The present invention relates to the field of communication and, more specifically, to the design of hybrid circuits and crosstalk cancellation techniques in bidirectional transmission.
The implementation of bidirectional transmission means allows to increase significantly the capacity of a communication system and therefore to bring a huge interest to the companies dealing with communication networks and to communication service providers. Nevertheless, the use of such transmission means implies to deal with additional technical problems. One of the main problems, in the case of a wire transmission, is the crosstalk phenomenon appearing at the ends of said wire and causing the degradation of said signals. Indeed, as the same wires are used to transmit and receive signals, some means must be employed to separate the strong near-end transmitted signal from the weaker far-end received signal. Circuits designed to separate the received signal from the transmitted signal are referred to as hybrid circuits.
FIG. 1 illustrates a wire connection between two hybrid transceivers. A first hybrid transceiver 1 comprising a hybrid circuit 2 generates and transmits a first signal V1 (V1 corresponds to the data signal Vt1 to be transmitted) and receives a different signal V2 (V2 corresponds to the signal Vt2 generated and transmitted from a remote hybrid transceiver 5) on the same wireline 3.
In order to obtain the signal voltage Vr1 corresponding to the received signal V2, the hybrid transceiver 1 needs only to subtract its generated and transmitted signal V1 from the hybrid signal Vb (Vb=V1+V2) on the wireline 3.
In the same way, the second hybrid transceiver 5 having a hybrid circuit 6 transmits the signal Vt2 and retrieves the signal Vr2 corresponding to V1.
Such operation to separate the transmitted and received signals is for example carried out by a hybrid circuit according to the state of the art as presented in FIG. 2.
The hybrid signal Vb corresponding to V1+V2 is coupled to the positive input of a subtracter 7 whereas a signal corresponding to V1 is coupled to the negative input in order to retrieve a signal corresponding to the received signal V2 on the output Vr1.
The impedance values of the different analog components (Ra, Ze) are chosen with respect to the line impedance (Zi).
The main problem is that the impedance of these components are fixed whereas the line impedance Zi of the line may vary a lot depending on transmission parameters (length of the line, wireline type, . . . ). In such cases, the separation of the transmitted and the received signals is imperfect leading to residual contributions of the transmitted signal in the received signal which is known as Near-end crosstalk (NEXT). Near-end crosstalk (NEXT) results therefore from transmitting and receiving different interfering signals on a wireline 3. The wireline said above comprises the media such as twisted pair (TP), coaxial link, microstrip or stripline on printed board.
In order to improve signal transmission, either a compromise has to be found on said transmission parameters or adjustments on the components of the local hybrid equipment have to be made. In most cases, the adjustment is done once during manufacturing, then this adjustment has to allow having enough margin to take into account the derating of environment parameters like for example values of temperature or supply. A more complex approach proposes techniques using digital adaptive filter that have been developed. However, to determine the coefficient of these filters, said techniques require external dedicated calculators and are, therefore, expensive and difficult to implement.